The communication environment is evolving towards increasingly heterogeneous but interconnected networks. The growth of demand for existing services and the introduction of new advanced services is expected to create a large increase of traffic flow in the near future. The current evolution of telecommunication network is led by asynchronous and synchronous transfer modes (Asynchronous Transfer Mode(ATM), Synchronous Optical Network (SONET), Synchronous Digital Hierarchy (SDH)), which require primarily electronic technologies for processing and switching. Although the necessary hardware building blocks are available to design wide area networks, complex issue arises with the management of network resources. In order to simplify the transfer task, the layer structure of the transport network and the use of optical means are preferred.
In all-optical networks, optical switching and routing become the most important issues for interconnecting the transport network layers. This invention describes a tunable optical add/drop filter for the optical WDM network applications. This filter can add or drop part of the high transmission capacity signals of a WDM link. It can be used to decentralized access point in the access network or as small core network node to realizing branching points in the network topology. It works in both wavelength and space domains.
The following describes various device structures that have been used for the add/drop filter design. The first structure [Cheung, "Acoustoopic Tunable Filters in Narrowband WDM networks: System Issues and Network Applications," IEEE J. Sele. Area Comm. 8(6), 1015, 1990.] uses four 1.times.N demultiplexers and N's 2.times.2 optical switches. The structure is complicated and the interconnections are difficult.
The second tunable add/drop filter, similar to the first geometry, has recently been proposed and demonstrated by Glance at AT&T. [Glance, "Tunable add/drop optical filter providing arbitrary channel arrangement", IEEE Photon. Lett., 7(11), 1303, 1995 and U.S. Pat. No. 5,488,500.] This filter provides the advantage of arbitrary channel arrangement, but still suffers a costly 6 dB optical coupling loss, because of the two array waveguide grating demultiplexers used in the structure.
The third type of wavelength-space switch [Dono et al, "A wavelength division multiple access network for computer communication", IEEE J. Sol. Area Comm., 8(6), 983, 1990.] has been widely used in various WDM networks, for example the IBM Rainbow Network. This structure uses a passive star-coupler that combines and splits the incoming light signals into N receivers. The receivers built with a tunable filter then select the desired channels. It has the broadcast capability and the control structure of this implementation is very simple. However, the undesirable feature of the broadcast star, the splitting loss can be very high when the users number is large.
The add/drop filter presented in this invention can re-route the unused channels, which are rejected by the tunable filter, back to the network and save the precious optical energy in the network. It includes the design of an optical isolator, that also dramatically cuts down the return loss of the device, another important performance requirement for high-speed WDM devices. It operates in both wavelength and space domains that provides hybrid functionality with a relatively simple structure. It is ideal for the WDM applications.